“Magnetic Particle Imaging” (MPI) is an emerging medical imaging modality. The first versions of MPI were two-dimensional in that they produced two-dimensional images. Future versions will be three-dimensional (3D). A time-dependent, or 4D, image of a non-static object can be created by combining a temporal sequence of 3D images to a movie, provided the object does not significantly change during the data acquisition for a single 3D image.
MPI is a reconstructive imaging method, like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Accordingly, a MP image of an object's volume of interest is generated in two steps. The first step, referred to as data acquisition, is performed using a MPI scanner. The MPI scanner has means to generate a static magnetic gradient field, called “selection field”, which has a single field free point (FFP) at the isocenter of the scanner. In addition, the scanner has means to generate a time-dependent, spatially nearly homogeneous magnetic field. Actually, this field is obtained by superimposing a rapidly changing field with a small amplitude, called “drive field”, and a slowly varying field with a large amplitude, called “focus field”. By adding the time-dependent drive and focus fields to the static selection field, the FFP may be moved along a predetermined FFP trajectory throughout a volume of scanning surrounding the isocenter. The scanner also has an arrangement of one or more, e.g. three, receive coils and can record any voltages induced in these coils. For the data acquisition, the object to be imaged is placed in the scanner such that the object's volume of interest is enclosed by the scanner's field of view, which is a subset of the volume of scanning.
The object must contain magnetic nanoparticles; if the object is an animal or a patient, a contrast agent containing such particles is administered to the animal or patient prior to the scan. During the data acquisition, the MPI scanner steers the FFP along a deliberately chosen trajectory that traces out the volume of scanning, or at least the field of view. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization. The changing magnetization of the nanoparticles induces a time dependent voltage in each of the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. The parameters that control the details of the data acquisition make up the scan protocol.
In the second step of the image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. The image is a discrete 3D array of data that represents a sampled approximation to the position-dependent concentration of the magnetic nanoparticles in the field of view. The reconstruction is generally performed by a computer, which executes a suitable computer program. Computer and computer program realize a reconstruction algorithm. The reconstruction algorithm is based on a mathematical model of the data acquisition. As with all reconstructive imaging methods, this model is an integral operator that acts on the acquired data; the reconstruction algorithm tries to undo, to the extent possible, the action of the model.
Such a MPI apparatus and method have the advantage that they can be used to examine arbitrary examination objects—e.g. human bodies—in a non-destructive manner and without causing any damage and with a high spatial resolution, both close to the surface and remote from the surface of the examination object. Such an arrangement and method are generally known and are first described in DE 101 51 778 A1 and in Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in nature, vol. 435, pp. 1214-1217. The arrangement and method for magnetic particle imaging (MPI) described in that publication take advantage of the non-linear magnetization curve of small magnetic particles.
As already explained, spatial encoding in MPI is based on the motion of a field free point or region over an object of interest. Thereby, the spectral response of the magnetic nanoparticles changes with distance to the FFP. High spectral components of said spectral response only occur in close vicinity to the FFP path or trajectory, whereas low spectral components are spatially rather delocalized. In certain situations problems may occur arising from delocalized low spectral components of the spectral response of the magnetic nanoparticles. For example in case of only encoding a sub-volume of a larger object or in case of completely encoding a larger object by splitting up the spatial encoding process. Splitting up the spatial encoding process means splitting up the large object into small sub-volumes, conducting encoding and reconstruction of a sub-volume image for each sub-volume separately and combining the sub-volume images for obtaining an overall image of the whole large object. In such situations signals from outside a specific sub-volume for which an encoding is conducted contribute to the low frequency components of the spectral response originated from those magnetic nanoparticles arranged in the specific sub-volume. Said signals from outside lead to a falsifications in the particle distribution quantity reconstructed for the specific sub-volume and therefore to artifacts in the sub-volume image reconstructed for the specific sub-volume. As the spectral response of the magnetic nanoparticles to the motion of the FFP is not completely localized, especially for low frequency components existing because of the encoding of the specific sub-volume, signals from outside the specific sub-volume are picked up.
The above-mentioned problems also may arise in case of leaving out sub-volumes of the field of view for speeding up the encoding process, wherein such sub-volumes are left out that cover areas which are not of interest to the operator of the MPI apparatus or MPI scanner. This approach is applied to situations at which in addition to the object of interest further objects are covered by the field of view. A concrete example for applying such an approach is the field of cardiac exam. With cardiac exam only subfields covering the region of the heart are encoded, while subfields covering for instance the liver are not encoded.
Because of the above described artifacts occurring in certain situations in reconstructed images existing MPI apparatuses and corresponding methods are still not optimal.